Based off the srt encoder. The following features are unimplemented:

- fonts, colors, sizes
- alignment and positioning

The rest works well. For example, use ffmpeg to convert subtitles into the .vtt format:

  ffmpeg -i input.srt output.vtt
Signed-off-by: Aman Gupta <ffmpeg@tmm1.net>
Signed-off-by: Clément Bœsch <u@pkh.me>